This invention relates to a procedure and a device for the semi-blind separation and detection of digital co-channel signals. Cellular mobile communication networks are usually limited by co-channel interference, i.e. the spatial reuse of the very same radio link becomes limited through co-channel interference. A radio link is defined by its frequency and/or its time slot (in the time-sharing multiplex method) and/or its code (in the code multiplex method). To allow more than one subscriber on the very same radio traffic channel, procedures have been proposed which are based on spatial separability and separation of the subscriber signals (SDMA, Space Division Multiple Access). For this purpose co-channel signals are purposefully generated and detected by the base station.
These procedures employ group antennas with connected signal processing, which by forming the antenna radiation pattern reduces the co-channel interference for individual subscribers. Such a procedure is for instance described in U.S. Pat. No. 5,515,378 (Roy and Ottersten). Three fundamentally different methods can be distinguished. Those that are based on knowledge of the spatial structure of the antenna group (the so-called spatial-reference-procedures) as described in R. Roy and R. Kailath in xe2x80x9cESPRIT-Estimation of Signal Parameters via Rotational Invariance Techniquesxe2x80x9d, IEEE Trans. Acoust., Speech, Signal Processing, Bd. 37, July 1989, pp. 984-995; procedures that are based on the knowledge of the known signal sequence (so-called temporal reference procedure), explained by S. Ratnavel, A. Paulraj and A. G. Constantinides in xe2x80x9cMMSE Space-Time Equalization for GSM Cellular Systemsxe2x80x9d, Proc. Institute of electrical and Electronics engineers, IEEE, Vehicular Technology Conference 1996, VTC""96, Atlanta, Ga., pp. 331-335; and so-called xe2x80x9cblindxe2x80x9d procedures which use known signal properties for separation and detection.
In the case of spatial procedures, the angle of incidence of the separate signals by different subscribers is estimated with the help of extensive procedures, and the information obtained is used to optimize the combination of the antenna signals. These spatially filtered signals are then detected in a separate receiver. Currently known methods have a lower limit for the angular separability and fail below this limit. This is a difficult problem because ideally the lower limit should be zero degrees.
A serious problem with the temporal reference procedure is the requirement to exactly synchronize the known training sequence with the incoming signals from a subscriber. Due to the multiple paths and the unknown distance between the transmitter and the receiver, the precise sampling instant is not known beforehand, and it also differs for the individual test signals. Only after synchronization can procedures be applied to adapt the weighting parameters of the individual antenna elements, which finally determines the effective radiation pattern. The alternative, the simultaneous synchronization and adapting of the weighting antenna parameters is computationally considerably extensive as shown in J. Fuhl, D. J. Cichon, E. Bonek, Optimum Antenna Topologies and Adaptation Strategies for SDMA, IEEE Global Communications Conference, Nov. 18-22, 1996, London, UK, PP. 575-580.
As far as the xe2x80x9cblindxe2x80x9d procedures are concerned, these use very common signal properties such as transmitting with a fixed sampling rate (FSR), selecting the symbols to be transmitted from a finite alphabet size (finite alphabet, FA) and transmitting the signals with a constant envelope (constant modulus, CM), as explained in J. Laurila, E. Bonek, SDMA Using Blind Adaptation, ACTS Mobile Communication Summit, Aalborg, Denmark, Oct. 7-10, 1997, pp. 314-319, or A-J. van der Veen, A. Paulraj, Singular Value Analysis of Time-Space Equalization in th GSM Mobile System, Proc. IEEE ICASSP""96, May 1996, Atlanta, Ga., pp. 1073-1076. These are computationally very extensive and have therefore not been used in the civilian mobile radio so far. The ILSP (Iterative Least Squares with Projection) procedure described in the publication by S. Talwar, M. Viberg, A. Paulraj called xe2x80x9cBlind Separation of Synchronous Co-Channel Digital Signals Using an Antenna Array-Part 1: Algorithmsxe2x80x9d. IEEE Trans. Signal Proc., Vol 44, pp. 1184-1197, May 1996, is based on the assumption of a finite number of transmission symbols, and is in principle suitable for the blind separation and detection of co-channel signals. It is, however, highly dependent on the accuracy of the initial values of the iterative signal estimation. The problem gets still worse when non-linear modulation methods are used for the transmission. This is unfortunately the case with modern mobile radio systems, for example in the GSM system (Global Systems for Mobile Communications) and in the DECT system (Digital Enhanced Cordless Telecommunications), which employ the non-linear Gaussian filtered Minimum Shift Keying modulation method. In such cases the above mentioned procedure must be complemented with an algorithm that takes advantage of the constant envelope of the signal, such as for instance ACMA (Analytical Constant Modulus Algorithm) proposed by A. J. van der Veen, A Paulraj in xe2x80x9cAn Analytical Constant Modulus Algorithmxe2x80x9d, IEEE Trans. on Signal Proc., vol 44, pp. 1136-1155, May 1996. This requires further extensive arithmetic steps added to the blind detection and signal separation procedure, which in itself is already very complex.
U.S. Pat. No. 5,619,533 (Dent) and U.S. Pat. No. 5,519,727 (Okanoue) describe an equalizer which employs a MLSE (Maximum Likelihood Sequence Estimator) that obtains its information from a sole antenna signal. A space-time equalization is not possible as there is only one antenna signal, but on account of the enormous amount of memory needed for the intermediate steps of the MLSE, it would anyhow be disadvantageous. The patent specification U.S. Pat. No. 5,432,816 (Gozzo) refers to a correction that together with the method of the least squares error provides recursive estimations of the channel. This equalizer method also only obtains its information from a sole antenna signal. Receivers that employ such equalizers are based on inaccurate channel estimation values, because they for instance cannot utilize any structural properties of the transmitted signals.
The invention is based on the requirement to provide a semi-blind procedure for the separation and detection of digital co-channel signals, comprising the steps:
simultaneous (joint) spatial-temporal equalization,
initialization of iterative projections using the subscriber identifier, and carrying out the above mentioned projections for the adaptation of a symbol vector both to the signal subspace and a finite alphabet symbol constellation, in both cases making use of the least squares error algorithm, but performing only matrix multiplications without eigenvalue or singular value decompositions.
The last step is repeated k times until the last iteration of the detected symbol vector no longer differs from the penultimate one.
The temporal synchronization for the initialization of the second step is easily achieved by correlating the respective subscriber identification with the basic vectors that span the desired vector subspace.
Another aspect of the invention is performing the projections in a decoupled way, dividing them between the individual subscribers. This allows the use of only simple matrix multiplication in these projections and avoids the excessive computing needed in traditional procedures. The extensive calculation of the pseudoinverse of the input matrix, including the orthonormal basic vectors of the desired vector subspace, is only required one single time, namely before initialization.
The temporal equalization carried out simultaneously with the spatial equalization, which is based on the processing of several antenna signals, leads to a co-channel interference suppression distinctly improved from that which the temporal equalization of one antenna signal alone can provide.